This invention relates to a method for regenerating an exhaust gas purification unit arranged in an internal combustion engine system. In particular, the invention relates to a method for regenerating a diesel particulate filter in a vehicle engine system in low-load or idling situations. The invention also relates to an internal combustion system adapted to be operated according to the method.
Exhaust gas from internal combustion engines contains hazardous compounds in the form of hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and/or particulate matter (PM). The exhaust gas is therefore normally led through an after-treatment system adapted to purify the exhaust gas. Such systems include for instance catalytic converters, NOx-traps and particle filters. Exhaust gas after-treatment components normally need to be operated within a certain temperature interval; if the temperature is too low the desired chemical reactions do not take place or the reaction rate becomes too low, and if the temperature is too high the components might be destroyed. Further, some components, such as NOx-traps and particle filters, need to be regenerated at a certain interval in order to work properly. Such regenerations are normally carried out at an increased temperature level.
With regard to internal combustion engines in vehicle applications the physical and chemical properties of the exhaust gas leaving the engine mainly depend on the type of engine and the conditions under which the vehicle is run. For instance, diesel engines generally produce larger amounts of particles and a cooler exhaust gas than an Otto-engine. Further, an engine that runs under high-load conditions generates exhaust gas with a much higher temperature than an idling engine. In order to perform a regeneration process, it may be necessary to take particular actions in the after-treatment system to further increase the temperature.
The exhaust gas properties of an engine operated at a high load for a longer time, such as an engine of a truck used for road transports, are normally suitable for regeneration purposes. However, a typical example where unfavourable exhaust gas properties causes problem in the after-treatment system is cleaning of the diesel particle filter of a refuse removal truck. In such a vehicle application, significant time is spent on idle speed with short, intermittent and limited driving distances, resulting in a relatively cool exhaust gas. To clean such a filter by injecting fuel and burning the particles, the temperature of the exhaust gas leaving the engine should normally be at least around 250° C.
A known way of increasing the temperature of the exhaust gas leaving the engine is to increase the amounts of fuel injected to the engine cylinders or to delay the injection (post-injection). A sufficiently high temperature is, however, normally not reached with such a measure alone.
With regard to regeneration of particle filters in low-load or idling situations a known method is to use a separate burner for heating up the filter. This is, however, a rather complicated solution since it requires additional equipment such as the burner itself and an air compressor.
US2005/0148430 discloses a method for increasing the exhaust gas temperature during low-load or idling operation wherein the engine load is increased by activating a brake and/or a starting element, such as a clutch or a torque converter. However, this method is not applicable in all situations since the parameters'affecting the exhaust gas properties are not fully considered.
It is desirable to provide a method and system for regeneration of a purification unit, which method and system create improved regeneration conditions in low-load or idling situations compared to known solutions.
An aspect of the invention concerns a method for regenerating an exhaust gas purification unit arranged in an internal combustion engine system, and the an aspect of the invention is characterized in that the method comprises the step of: setting a predefined minimum engine speed such that the exhaust gas mass flow exceeds a predefined flow value during the regeneration process.
An aspect of the inventive method has the advantageous effect of avoiding regeneration of the purification unit in situations wherein the purification unit may be destroyed if subjected to regeneration because of a too low exhaust gas mass flow. For instance, a conventional diesel particulate filter is likely to be destroyed by overheating if regenerated when the exhaust gas mass flow is below a certain level. Moreover, the inventive method has the advantageous effect of establishing suitable conditions for initiating or maintaining regeneration of the purification unit in situations where the exhaust mass flow is too low, such as in low-load or idling situations. The use of a minimum engine speed for ensuring that the exhaust gas mass flow becomes sufficient is useful regardless of whether any special actions are taken to increase or control the exhaust gas temperature.
In a preferred embodiment of an aspect of the invention the method also comprises the step of setting a predefined minimum engine torque such that the exhaust gas temperature exceeds a predefined temperature value during the regeneration process. A regeneration process can in such a case be performed when required, i.e. it is not necessary to delay the start of, or to interrupt, the regeneration because of an unfavourable exhaust gas mass flow or temperature.
In a further preferred embodiment of an aspect of the invention, the predefined minimum engine speed and the predefined minimum engine torque are set in combination such that the exhaust gas mass flow exceeds a predefined flow value and the exhaust gas temperature exceeds a predefined temperature value during the regeneration process. This reflects e.g. a case where a change of the engine speed has a direct effect on the engine torque, such as when a hydraulic torque converter is connected to the engine. In addition, any influence of the engine speed to the exhaust gas temperature, and of the engine torque to the exhaust gas mass flow, can thereby be taken into account.
In a further preferred embodiment of an aspect of the invention, the method further comprises one or several of the following steps: determining whether a regeneration of said exhaust gas purification unit is required, determining the exhaust gas temperature, and performing regeneration of said exhaust gas purification unit in case the exhaust gas mass flow exceeds said predefined flow value.
In a further preferred embodiment of an aspect of the invention, the engine system comprises an internal combustion engine; a gearbox; a starting element arranged to connect the engine and the gearbox, said starting element being capable of slipping; and a braking element capable of increasing the engine load when activated, wherein the method further comprises the steps of: activating the braking element, determining whether a gear is engaged and, if that is not the case, engaging a gear. This reflects the typical case where a refuse removal truck equipped with a hydraulic torque converter and an automatic gearbox operates in a low-load situation.
An aspect of the invention also concerns an internal combustion engine system comprising an exhaust gas purification unit, wherein the system is adapted to be operated according to the method described above.